Existing video image coding technologies include an intra-frame coding technology and an inter-frame coding technology. The intra-frame coding refers to a technology of coding image content by using only a spatial correlation within a current image block that is being coded. The inter-frame coding refers to a technology of coding a current image block by using a time correlation between the current image block that is being coded and an image block that has been coded.
In order to increase intra-frame coding efficiency of an image, an H.264/AVC standard first introduces an intra-frame prediction technology to remove spatial information redundancy between a current image that is being coded (hereinafter referred to as a current block) and a neighboring coded image block. An HEVC solution is a new-generation standardized video coding solution currently being researched by the International Organization for Standardization. This solution inherits and expands an intra-frame prediction coding technology in the H.264/AVC standard. In the HEVC solution, a prediction mode set of an image block chrominance component may include six optional prediction modes:
DM mode: performing prediction by using a prediction mode for a luminance component of a current block as a prediction mode for a chrominance component of the current block;
LM mode: computing a prediction value of a chrominance component sampling point by using a reconstruction value of a luminance component sampling point based on a correlation model, where a parameter of the correlation model is obtained by performing computation according to reconstruction values of luminance component sampling points and chrominance component sampling points directly above and directly to the left of a current block;
DC mode: using an average value of reconstruction values of neighboring chrominance component sampling points directly above and directly to the left of a current block as a prediction value of a chrominance component sampling point of the current block;
Planar (Planar) mode: computing a prediction value of a chrominance component sampling point of a current block based on an assumption about a linear smooth variation of a value of a chrominance component sampling point in space;
Horizontal mode: using a reconstruction value of a neighboring chrominance component sampling point directly to the left of a current block as a prediction value of all chrominance component sampling points within a same row in the current block; and
Vertical mode: using a reconstruction value of a neighboring chrominance component sampling point directly above a current block as a prediction value of all chrominance component sampling points within a same column in the current block.
The DC mode, the vertical mode, the horizontal mode, and the planar mode in the foregoing prediction modes and corresponding prediction modes in the H.264/AVC standard are the same in basic principles but different in specific implementation methods. The LM mode and the DM mode are two newly added prediction modes. Besides the foregoing prediction modes, a backup mode also exists. If the prediction mode for the current block luminance component is the same as a certain prediction mode among the rest of the modes in the prediction mode set, the prediction mode set of the chrominance component includes two identical prediction modes, namely, the DM mode and the certain prediction mode. In this case, the backup mode is used to replace the certain prediction mode, thereby forming a new prediction mode set in which no prediction modes are the same. The backup mode may be a prediction mode different from all the prediction modes in the prediction mode set of the chrominance component; for example, the backup mode may be a diagonal mode in which prediction is performed in a diagonal direction.
However, in the existing HEVC solution, selection of the prediction mode set of the chrominance component causes high complexity of implementing a decoding end.